Recovery of Chemicals and Fuels through the Thermo-Chemical Process of Municipal Solid Wastes

Abstract

A rapid increase in the population growth and the urbanization has resulted in the massive generation of MSW. Conventional practices (e.g., landfilling and incineration) for the disposal of MSW have posed severe environmental problems such as discharges of microplastics and/or toxic pollutants. As a precautionary measure to attenuate the potential hazards, seeking an environmentally benign disposal for MSW is of importance. It is more desirable to convert MSW into value-added chemicals and fuels. Nevertheless, MSW is a mixture comprising various (in)organic materials. The composition of MSW is strongly influenced by the consumers’ patterns. Such heterogeneous nature of MSW deteriorates its valorization into desirable products. In this context, the thermo-chemical process (pyrolysis) could be a promising option to tolerate the heterogeneous nature of MSW. This practice is an effective way transforming carbon in MSW into three phased pyrogenic products. Nevertheless, prior to the direct conversion of MSW, an investigation of the pyrogenic products stemming from the single wastes (such as plastics and biomass) is preferentially recommended as a strategic way to avoid any complexities arising from the intricate composition of MSW. This is reasonable because the distribution of the pyrogenic products is contingent on the chemical composition of specific wastes. Thus, this thesis laid a great stress on the thermo-chemical conversion of plastics/biomass into value-added products. In an effort to impart a sustainability to the pyrolysis system, CO2 was employed as a reactive gas medium. Investigation on the functionality of CO2 to react with the pyrogenic products evolved from plastics/biomass was carried out throughout this thesis. To this end, this thesis is arranged with five specific chapters as below. Chapter 2 proposed the sustainable valorization of automotive bumper waste (mainly comprising polypropylene (PP)) into syngas. Pyrolysis of PP resulted in wide ranged carbon distribution having C8-C46 aliphatic HCs. Catalytic pyrolysis using Co/SiO2 and Ni/SiO2 led to an increased production of H2 with coke formation over the catalysts. The use of CO2 led to additional formation of CO and suppressed formation of coke. The molar ratio of resulting syngas was altered in the range of 15 to 0.7 by controlling the concentrations of CO2 as purge gas in the catalytic pyrolysis. Chapter 3 tested the thermo-chemical conversion of polystyrene (PS) into monomer or syngas. Pyrolysis oil stemming from PS includes styrene monomer, styrene dimer, and styrene trimer. A change in the compositional matrix of pyrolytic products was monitored as the function of reaction temperature. At 600 ˚C, PS pyrolysis showed the highest selectivity for styrene monomer. At ≥ 600 ˚C, the enhanced formation of polycyclic aromatic hydrocarbons (PAHs) was identified. The use of CO2 enabled to the reduction of PAHs, in comparison with N2 condition. Chapter 4 focused on the functionality of CO2 for the mitigation of toxic pollutants liberated from the pyrolysis process of plastic waste having a high aromaticity. To this end, building insulator waste (polyurethane foam, PUR) was adopted as a model compound of aromatic-based plastics. PUR pyrolysis liberated the formation of hazardous chemicals (4,4′-methylenedianiline and its derivatives). The use of CO2 allowed the toxic pollutants to be transformed/detoxified into syngas. Indeed, 78.4 wt.% reduction of toxic pollutants and simultaneous conversion into syngas (mainly CO) was achieved. Chapter 5 conducted the thermo-chemical conversion of yard trimming (one of the main materials in MSW) under the CO2 environment. Employing CO2 helped to modify the product distribution from pyrolytic oil into syngas. To boost the reaction rates of CO2 for producing syngas, concrete (one of the inorganic materials in MSW) was utilized as the catalytic materials. The high content of Ca present in concrete facilitated the conversion of yard trimming/CO2 into syngas. Chapter 6 tested the practical applications of biochar produced from the pyrolysis of toxic microalgae. CO2-cofed catalytic pyrolysis of toxic microalgae (Microcystis aeruginosa) waste was conducted. Gaseous pyrolysates concentration and biochar surface properties were analyzed with a construction of mass balance. Also, the catalytic capability of biochar improving the mechanistic function of CO2 from the pyrolysis system was investigated. Ni/biochar composites were fabricated, and their practical applications as a catalyst to promote syngas formation were evaluated. KEYWORDS: Circular Economy; Waste Valorization; Thermo-Chemical Process; Municipal Solid Waste; CO2 Utilization